dont append <pre> if there are no errors to show

given command-line output:

```
200    3ms    1.24KB /flatui/images/toggle/icon-on.png
200 2441ms  847.18KB /bundle.js -> browserify ./index.js
200    1ms    1.22KB /flatui/images/toggle/icon-off.png
404    1ms        9B /favicon.ico
```
before patch:

![Screen Shot 2013-03-18 at 2 58 27 PM](https://f.cloud.github.com/assets/39759/273082/0f5b2b5c-9017-11e2-9f12-eae02b198833.png)

after patch:

![Screen Shot 2013-03-18 at 2 58 44 PM](https://f.cloud.github.com/assets/39759/273083/1421c038-9017-11e2-8581-04cbe3e6b84f.png)